Mouth sores develop when the soft tissue lining your mouth breaks down, whether from physical damage, infection, nutritional gaps, or an overactive immune response. Most are canker sores (small ulcers inside the mouth) or cold sores (blisters on or around the lips), and the vast majority heal on their own within one to two weeks. Understanding what triggered yours helps you prevent the next one.
Physical Injury Is the Most Common Trigger
The lining of your mouth is delicate, and everyday mechanical damage is the single most frequent reason sores appear. Biting your cheek or tongue, burning your mouth on hot food, or scraping the tissue with a sharp chip or cracker can all create a small ulcer that takes days to heal. Aggressive tooth brushing is another common culprit, especially if you use a hard-bristled brush.
Orthodontic appliances are a major source of mouth sores. About 95% of people with braces report pain during treatment, often from brackets and wires rubbing against the cheeks, lips, or tongue. The friction creates traumatic ulcers at the contact points. Ill-fitting dentures cause the same kind of repeated irritation, and even a single rough or broken tooth edge can produce sores that keep coming back until the source of friction is fixed.
Canker Sores vs. Cold Sores
These two get confused constantly, but they look different and have completely different causes.
Canker sores appear inside your mouth, on the inner cheeks, lips, tongue, or soft palate. They’re typically a single round sore with a white or yellow center and a red border. They are not contagious. Anywhere from 5% to 66% of the population gets them, depending on the group studied, and they can start in childhood, sometimes before age five.
Cold sores (fever blisters) show up on the outside of your mouth, usually around the lips. They look like a cluster of small, fluid-filled blisters rather than a single open ulcer. Cold sores are caused by herpes simplex virus, which lives inside nerve cells and cycles between inactive and active states. Once you’re infected, the virus stays in your body permanently. Reactivation triggers include illness or fever, sun exposure, emotional stress, menstrual periods, injury to the area, and surgery.
Nutritional Deficiencies That Weaken Mouth Tissue
Your mouth’s lining regenerates quickly, but it needs the right raw materials. Recurring mouth sores are often linked to low levels of vitamin B12, folate (B9), or iron. These nutrients support cell repair and tissue regeneration. When your body runs short, the oral lining becomes fragile and more sensitive to everyday irritation that wouldn’t normally cause a sore.
Each deficiency shows up slightly differently. Low iron reduces oxygen delivery to tissues, leading to pale gums and a sore tongue. B12 deficiency can cause a smooth, burning tongue along with ulcers. Folate deficiency impairs the mouth’s ability to regenerate damaged tissue. Riboflavin (B2) deficiency tends to cause cracked corners at the edges of the mouth rather than sores inside it. If you get mouth sores repeatedly and can’t identify an obvious trigger like braces or biting, a simple blood test can check these levels.
Chemical Irritants in Everyday Products
A foaming agent called sodium lauryl sulfate (SLS), found in many common toothpastes, can strip away the protective layers of your mouth’s lining. Research has shown that SLS toothpaste can increase the frequency of recurring mouth ulcers. One clinical study found four times fewer soft tissue lesions after using an SLS-free toothpaste compared to one containing SLS. If you get canker sores frequently, switching to an SLS-free toothpaste is one of the simplest changes you can make.
Acidic foods and drinks, including citrus fruits, tomatoes, and coffee, can also irritate existing sores or trigger new ones in people who are prone to them. Alcohol-based mouthwashes have a similar effect for some people.
Underlying Health Conditions
Sometimes frequent mouth sores are a sign of something deeper going on. Celiac disease is one of the clearest examples. People with celiac disease have more frequent and more severe outbreaks of canker sores, and following a strict gluten-free diet has been shown to reduce both the frequency and severity of those outbreaks. For some people, recurring mouth ulcers are actually the symptom that leads to a celiac diagnosis.
Crohn’s disease and other inflammatory bowel conditions can also cause oral ulcers, as can immune system disorders that disrupt the normal balance of inflammation in your body. Hormonal shifts, particularly around menstruation, trigger canker sores in some people. Emotional stress is another well-documented trigger, likely because stress hormones alter immune function in ways that make the mouth lining more vulnerable.
How Long Mouth Sores Take to Heal
Most mouth ulcers heal on their own without treatment in about one to two weeks. Minor canker sores, the most common type, are generally small and round, and they resolve closer to the one-week mark. Keeping the area clean and avoiding spicy, acidic, or crunchy foods helps prevent additional irritation while you wait.
Over-the-counter topical gels or rinses can reduce pain and protect the sore from further contact. For cold sores, antiviral treatments work best when applied at the first tingle, before blisters fully form.
Signs a Sore Needs Attention
A mouth sore that doesn’t heal within two weeks is worth getting checked. Oral cancer can look like a sore that simply won’t go away, a white or reddish patch inside the mouth, a lump or growth, or persistent pain. Other warning signs include loose teeth without an obvious dental cause, ear pain, and difficulty swallowing or opening your mouth. None of these symptoms automatically mean cancer. Infections and other treatable conditions can look similar. But the two-week mark is a useful threshold: if a sore hasn’t started healing by then, it’s no longer behaving like a typical ulcer.